of Metals for reflecting Telef copes. 339 
reflectors (whofe fpecula, unlefs by accident, are always 
fpherical) that the only true rays which form the dif- 
tinCt image arife from the middle of the metal : and un- 
lefs the defect be remedied by a confiderable aperture, 
which deftroys much light, the falfe reflection from the 
infide of the metal produces a greyifh kind of hazinefs, 
which is never feen in Mr. short’s or indeed in any 
good telelcopes. 
Suppofing that the two foci of the different parts of the 
metal perfectly coincide, and that, by the union of them 
when the apertures are removed, the telefcope thews 
the objects very fharp and diftinCt, you are not however 
even then to conclude that theinflrument is not capable of 
farther improvement; for you will perceive a fen Able 
difference in the fharpnefs of the image, under different 
pofitions of the great fpeculum with refpeCt to the little 
one, by turning round the great metal in its cell, and 
oppofing different parts of it to different parts of the little 
metal, correcting by this means the error of one by the 
other. This attempt fhould be perfevered in for fome 
time, turning round the great fpeculum about one-fix- 
teenth at a time, and carefully obferving the moft diftinCt 
fituation each time the eye-piece is fcrewed on : when, 
by trying and turning the great metal all round, the 
diftinCteft pofition is difcovered, the upper part of the 
X x a metal 
